1. Field of the Invention
The present invention relates to a data transmission rate arbitration method, and more particularly to a data transmission rate arbitration method for use in an optical communication system which decides an intra-device data transmission rate based on a transmission band of an optical transmission line.
2. Description of the Background Art
In recent years, digital devices have come into practical use, and digital networks have become increasingly more prevalent. The IEEE1394 standard provides an example interface for interconnecting digital devices. The IEEE1394 standard defines a number of data transmission rates from 100 Mbps to 3200 Mbps. According to the IEEE1394 standard, communications between devices can be performed in such a manner that data transmissions are allowed to occur between devices having different data transmission rates. In a digital network which allows data transmissions to occur at a plurality of data transmission rates, it is necessary to decide a data transmission rate to be used between devices (hereinafter referred to as the “intra-device data transmission rate”) prior to each data transmission. In a conventional digital network, an intra-device data transmission rate is decided based only on a data transmission rate which is acceptable to each device. Specifically, the information of a data transmission rate which is acceptable to each device is conveyed to another device by using a control signal, and the lower one of the two data transmission rates that are acceptable to the interested devices is designated as the data transmission rate between the two devices.
On the other hand, the IEEE1394 standard contemplates the use of optical fibers as an alternative transmission line to the conventional electrical cables, with a view to realizing a higher data transmission rate and enabling connection over longer distances. FIG. 10 is a block diagram illustrating the structure of a conventional optical communication system. Hereinafter, an arbitration operation for data transmission rates between devices, which is performed in the case where the conventional optical communication system shown in FIG. 10 is adopted in a network which permits the use of a plurality of data transmission rates, will be described.
The optical communication system comprises devices 91 and 92, which are coupled via an optical transmission line 93. The device 91 includes a storage section 911, a data transmission rate arbitration section 912, and an optical transmission/reception section 913. The device 92 includes a storage section 921, a data transmission rate arbitration section 922, and an optical transmission/reception section 923. A data transmission rate arbitration occurs as the data transmission rate arbitration sections 912 and 922 transmit and receive the information of the maximum data transmission rate which is acceptable to each device by using control signals. The control signals are signals used for transmitting information such as data transmission rates, detection of a connection, response to the detection of a connection, and notification of a completion of the data transmission rate arbitration. In general, low-frequency signals are used as the control signals. After detection of a connection, the data transmission rate arbitration section 912 in the device 91 transmits the information of the maximum data transmission rate acceptable to the device 91, which is stored in the storage section 911, to the device 92. Similarly, the data transmission rate arbitration section 922 in the device 92 transmits the information of the maximum data transmission rate acceptable to the device 92, which is stored in the storage section 921, to the device 91. The data transmission rate arbitration section 912 in the device 91 compares the maximum data transmission rate acceptable to the device 92, which has been transmitted from the device 92, against the maximum data transmission rate acceptable to the device 91, which is stored in the storage section 911. Based on the comparison, the data transmission rate arbitration section 912 transmits to the device 92 the information of the data transmission rate having a smaller value, by using a control signal. Thereafter, the data transmission rate arbitration sections 912 and 922 keep exchanging the data transmission rate information until the transmitted data transmission rate information and the received data transmission rate information reach the same value. Once the same value is reached by the respective data transmission rate information, the devices 91 and 92 add information notifying the completion of the data transmission rate arbitration to the data transmission rate information, and transmit a control signal, thereby completing the data transmission rate arbitration. Thus, it becomes possible for the devices 91 and 92 to perform data transmissions based on a data transmission rate which is compliant with both a data transmission rate which is acceptable to each device and a data transmission rate which is acceptable to each other device.
As mentioned above, it is possible to utilize an optical fiber as a transmission line. Depending on the purposes, various types of optical fibers having various transmission band characteristics are available. For example, based on the materials, optical fibers can be classified into GOFs (Glass Optical Fibers), PCFs (Polymer-clad Fibers), POFs (Plastic Optical Fibers), and the like. Based on another type of classification, optical fibers can be classified into step index (SI) fibers, graded index (GI) fibers, and the like. The transmission band characteristics of optical fibers may differ depending on the material and/or type of the optical fiber as listed above. The transmission band characteristics may also vary depending on the transmission distance over the optical fiber.
As described above, so many types of optical fibers are currently available. One implication of this is that two optical fibers which are apparently of the same connect shape may have different transmission bands. Stated otherwise, an optical fiber may be physically connectable without having satisfactory transmission band characteristics. Therefore, without knowing the actual transmission band of the optical fiber, a user may connect an optical fiber which does not satisfy desired transmission band characteristics. It is also possible for the same optical fiber to keep being used after a device to which the optical fiber is connected is changed. In such cases, too, the optical fiber may keep being used although not satisfying the desired transmission band characteristics.
As described above, it is possible that a given intra-device data transmission rate and the transmission band of a given optical transmission line may not match, resulting in an inadequate transmission band. Yet, the decision of the data transmission rate in a conventional optical communication system is based only on the data transmission rate which is acceptable to each device, i.e., without taking into account the transmission band of the optical transmission line. As a result, a conventional data transmission rate arbitration process may designate a data transmission rate which does not actually correspond to the transmission band of the optical transmission line. In such cases, data transmissions between devices cannot occur, resulting in a communication failure.
As an example, a case will be considered in which an optical fiber is employed as the optical transmission line 93 in the optical communication system shown in FIG. 9. It is assumed that the devices 91 and 92 both accept data transmission rates of 100 Mbps, 200 Mbps, and 400 Mbps, and that the optical fiber used has transmission band characteristics such that a signal can be transmitted therethrough at a data transmission rate of 200 Mbps or less. Under these assumptions, a conventional optical communication system would decide the data transmission rate based only on the data transmission rate which is acceptable to the devices 91 and 92, and therefore set the data transmission rate to be 400 Mbps. However, the optical fiber can only transmit signals therethrough at a data transmission rate of 200 Mbps or less. Thus, the transmission band of the optical fiber is inadequate with respect to the decided data transmission rate, making it impossible to perform communications between the devices 91 and 92.